We carried out a case-control study of lung cancer risk associated with residental exposure to radon. The study measured radon levels in all adult residences as well as in a significant childhood residences and attempted to take into account exposure variations within a home and over time. The study included men and women and both smokers and non-smokers in order to explore possible interactions between smoking and radon. The study includes 1474 cases and 1811 controls. Only 129 of the lung cancer cases had never smoked, but most were nonsmokers at the time of cancer diagnosis. The average radon exposure levels were low; the average level in measured homes was 1.5 pCi/L in Utah and 0.9 pCi/L in Connecticut. Only 3% of Connecticut homes and 7% of Utah homes exceeded the US EPA's "action level" of 4 pCi/L. We calculated cumulative and average radon exposure estimates for all participants. The excess relative risk associated with radon was estimated after controlling for decade-specific smoking and other relevant variables and interactions. While our results appear to be consistent with those of other studies - including extrapolations based on miners, there is little evidence of any excess relative risk of lung cancer associated with radon in this study. Estimates were greater when data were restricted to participants who did not require proxy data, to those with complete coverage (i.e no imputed data) during the 20-year exposure window of interest, to nonsmokers, or to Connecticut, but no result is statistically significant. We found an increase in lung cancer risk associated increasing average exposure to TSP. Risk estimates were greater in Connecticut, where TSP may be a better marker of carcinogenic agents in air pollution than in Utah. Lung cancer risk increased with increasing level of TSP. The effects of smoking and TSP appeared to be multiplicative. Radon data from this study have been pooled with data from other case-control studies in North America for a more precise estimate of risk associated with low-dose residential radon exposure.